1. Field of the Invention
This invention relates to pressure molding systems, which are used, for example, to mold articles such as closures for containers. More specifically, this invention relates to improvements in pressure molding systems that increase their operating life and reliability.
2. Description of the Related Technology
Pressure molding systems typically include a source or sources of hot flowable plastic material, a number of mold cavities, and at least one distribution manifold system or xe2x80x9chot runnerxe2x80x9d for supplying the hot flowable plastic material to the mold cavities. This is done via nozzles under conditions of controlled pressure and temperature, and with substantially uniform flow rates, thereby creating uniform quality molded products in each of the mold cavities.
FIGS. 1 and 2 depict one commercially available pressure molding system, which is described in U.S. Pat. No. 4,588,367 to Schad. As may be seen in FIGS. 1 and 2, a floating manifold 11 is disposed between thermal expansion support elements 12, 13 that are received in mating wells 14, 16 formed in a manifold plate 17 and a backing plate 18, respectively. A pressure molding system 19 advances plastic material under high pressure through a conduit 21 into a main manifold channel 22 and thence to nozzles, 23 and 24, which, in turn, direct plastic into mold, cavities 26 and 27 formed in platens 28 and 29.
As shown in FIG. 2, the thermal expansion element 12 includes a tubular member 32 that surrounds and is spaced from a nozzle 24. One end of the tubular member 32 is connected to the body of the expansion element by a bridge 33, and the other end terminates in a peripheral flange 34, which fits within the sidewall 36 of well 14. On the opposite side of manifold 11, well 16 formed in abutment 18 receives a support element 13. Element 13, independent of a nozzle, is seated in the bottom of well 16 and its head 43 contacts one side of floating manifold 11 as does head 44 of combined nozzle 24 and thermal expansion support element 12 upon the opposite side of the manifold.
As temperature builds up during the course of machine start-up, the manifold 11 conducts heat to nozzle 24. In addition, the nozzle 24 is heated by electric resistance heaters 50 that are supplied electricity by a plurality of electric wires. The manifold 11 tends to expand as the temperature increases and the tubular members 32-32a of support elements 12 and 13 also expand creating compressive forces to effect a seal at the interface of each nozzle and its mating manifold orifice.
As may be seen in FIG. 3, the nozzle 23 is disposed within a nozzle cavity 52 that is defined in the mold platen 28. In operation, nozzle cavity 52 fills with hot flowable plastic material 58, which is forced out of the gate 56 into the mold cavity 26 as it is displaced by additional hot flowable plastic material that is introduced from the nozzle 23. A shut-off seal 54 is provided to seal or shut off the nozzle cavity 52 from the space in which, among other things, the electric resistance heater is disposed. Shut-off seal 54 is typically made of a metallic material, such as steel, and is sized so as to press tightly against the wall 60 defining a lower end of the nozzle cavity 52.
Unfortunately, in practice it has been determined that the shut-off seal 54 is insufficiently effective in containing the hot flowable plastic material within the nozzle cavity 52 during operation. This may be due to a number of factors, such as damage that can occur to the seals 54 and the walls 60 during routine maintenance and mold changeovers. Regardless of the reasons, the result of leakage from the seals 54 is that plastic material ends up in unwanted areas of the manifold structure, and in particular in the area of the electric resistance heaters 50. This has been found to disable individual heaters 50, in particular by fouling the electrical wires, which results in the entire mold freezing up and becoming disabled.
A need exists for an improved system and method for increasing the reliability and operating life of pressure molding systems of the type that have been described above in reference to FIGS. 1-3.
Accordingly, it is an object of the invention to provide an improved system and method for increasing the reliability and operating life of pressure molding systems of the type that have been described above in reference to FIGS. 1-3.
In order to achieve the above and other objects of the invention, a system for pressure molding plastic material includes, according to a first aspect of the invention, a source of hot flowable plastic material; a manifold for receiving hot flowable plastic material from the source and for transporting the hot flowable plastic material; a nozzle cavity that is defined by at least one wall; a nozzle disposed within the nozzle cavity, the nozzle being operatively in communication with the manifold for receiving hot flowable plastic material therefrom; a heater for heating the nozzle, the heater being positioned outside of, but close to, the nozzle cavity and having at least one electrical supply wire; a shut-off seal positioned within the nozzle cavity to seal the nozzle with respect to the wall defining the shut-off cavity; and a secondary seal positioned between the shut-off seal and the electrical supply wire for ensuring that any hot flowable plastic material that might leak through the shut-off seal does not come into contact with the heater.
According to a second aspect of the invention, a nozzle assembly for pressure molding plastic material, includes a nozzle cavity that is defined by at least one wall; a nozzle disposed within the nozzle cavity, the nozzle being constructed and arranged to receive hot flowable plastic material from a source thereof; a heater for heating the nozzle, the heater being positioned outside of, but close to, the nozzle cavity and including an electrical supply wire; a shut-off seal positioned within the nozzle cavity to seal the nozzle with respect to the wall defining the shut-off cavity; and a secondary seal positioned between the shut-off seal and the heater for ensuring that any hot flowable plastic material that might leak through the shut-off seal does not come into contact with the electrical supply wire.
According to a third aspect of the invention, a method of retrofitting a pressure molding system of the type that includes a nozzle cavity having a nozzle disposed therein that is constructed and arranged to receive hot flowable plastic material from a source thereof, a heater for heating the nozzle that is positioned outside of, but close to, the nozzle cavity and includes an electrical supply wire, and a shut-off seal positioned within the nozzle cavity to seal the nozzle with respect to the wall defining the shut-off cavity, includes steps of gaining access to an area within the pressure molding system that is between the shut-off seal and the heater; and installing a secondary seal in the area between the shut-off seal and the heater to ensure that any hot flowable plastic material that might leak through the shut-off seal does not come into contact with the electrical supply wire.
These and various other advantages and features of novelty that characterize the invention are pointed out with particularity in the claims annexed hereto and forming a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to the accompanying descriptive matter, in which there is illustrated and described a preferred embodiment of the invention.